In various situations it is necessary to measure distances unobtrusively between two points (i.e. there must be no physical members connecting the points), or to measure changes in distances between the two points. It is often necessary for these measurements to be fully automated and to be made continuously or at regular intervals. One particular example where regular unobtrusive distance and displacement measurements are required, is in the underground mining industry. Underground mine shafts are prone to collapse, resulting in significant losses in productivity and possibly lives. Usually collapses of underground shafts are preceded by the convergence of the shaft roof and floor. By detecting this convergence, it is possible to predict impending shaft collapses, and thus allows mining operators to attempt to avoid a possible disaster.
It is preferable that such a divergence detection system should have a high degree of accuracy, reliability and repeatability under conditions which are adverse while being unobtrusive and easily transportable. Existing measurement systems include extensiometers, wire-wound potentiometers and laser interferometers.
Extensiometers and wire-wound potentiometers suffer from a number of limitations. They rely on an obtrusive measurement technique resulting in errors due to mechanical disturbances arising from the general nature of mining operations. They are often used only as temporary apparatus and have limited resolution. For these reasons they cannot be used in many areas where convergence measurements are required. Laser interferometry suffers from the fact that a relatively clean environment is necessary for correct operation. The environment within a mine is in direct conflict with this requirement. Furthermore, its cost make laser systems very undesirable.
The present invention attempts to overcome one or more of the above disadvantages with the use of ultrasonic waves, ultrasound. Ultrasound is comprised of travelling longitudinal mechanical waves at frequencies above those audible to the human ear, normally above twenty kilohertz. When travelling through air, the waves may be described in terms of the variation of air pressure at a particular point. The pressure varies with simple harmonic motion, firstly above and then below the average atmospheric pressure at that point. The reflection of ultrasonic waves from a plain surface is similar to the reflection of light from a non-ideal mirror. That is, the angle of reflection is approximately equal to the angle of incidence. This is especially true at high frequencies. However, as the frequencies decrease, more defraction and dispersion take place.
Distance between and displacement of two points in space can be measured using ultrasound measurements. By comparing distance measurements at different points in time, it is possible to detect relative movement between the points in space. In order to measure the distance between the points, an ultrasonic toneburst can be projected from one of the points using an ultrasonic transducer, the toneburst is reflected by a suitable reflector at the second point and the toneburst then returns to the point of transmission. The transit time of the pulse is proportional to the total distance travelled. It is critical to accurately measure the time between the transmission and reception of the toneburst, in order to provide an accurate measurement of the distance between the two points.
In order to complete the distance measurement it is necessary to have knowledge of the speed of the ultrasonic waves in air. This speed may vary depending on temperature, air pressure, moisture content, etc. If it is known that the speed of sound does not change in the application of the UDMS, then a constant value for the speed of sound may be used. Alternatively, if the speed of sound does change, then a measurement of the current speed of sound is required.
FIGS. 1 and 2 illustrate known ultrasonic distance measurement systems (UDMS). FIG. 1 shows a UDMS unit 1 at a first point in space having a transmitting transducer 2, a receiving transducer 3 and a reflector 4 at a second point in space. The UDMS use known methods to determine the time required for an ultrasonic signal to travel the unknown distance d1 between the two points in space. If it is known that the speed of sound is variable over time around the UDMS unit 1, the UDMS 1 may also be provided with a subsystem 5 for measuring the current temperature from which it is possible to determine the speed of sound. Using the transit time period required for a signal to travel between the two points and a fixed or measured value for the speed of sound, it is possible to calculate the distance d1 between the two points.
FIG. 2 illustrates a UDMS 1 having a subsystem 5 comprised of transmitting transducer 6 and receiving transducer 7 at a known distance d2 from each other. From a measurement of the transit time over the known distance d2 it is possible to calculate the speed of sound. From a measurement of the transit time between the UDMS 1 and the reflector 4, and the calculated speed of sound, it is possible to determine the distance d1 between the two points.
The intensity of an ultrasonic wave in air attenuates at approximately inverse parabolic function against distance travelled. Furthermore, the attenuation becomes more rapid at higher frequencies. It is for this reason that ultrasonic ranging systems use frequencies in the range 20 to 200 kilohertz when measurements greater than a few meters are required. These frequencies correspond to wave lengths of 70 millimeters to 1.7 millimeters, respectively, in air. At this point it should also be noted that the spacial intensity distribution is a direct function of the frequency being transmitted. Higher frequencies result in a reasonably directed output but suffer from rapid attenuation, whilst very low frequencies result in less attenuation but a much more hemispherical distribution from an ultrasonic transducer. It is a problem to find a particular operating frequency which has an acceptable degree of attenuation and directionality for reliable operation of a UDMS over a wide range of distances.
Ultrasonic transducer convert electrical impulses to mechanical ultrasonic waves upon transmission, and vice versa when they are received by the transducers. The most commonly available ultrasonic transducers are of a piezoelectric or electrostatic type. As with most transducers, these are not ideal. One of the most significant problems with transducers is that the transfer from electrical to mechanical, or mechanical to electrical waves, is not instantaneous.
The main problem with this is that the amplitude of the ultrasonic wave as it is transferred to air is not constant. Many cycles of the tone burst are required before the amplitude of the mechanical wave reaches its maximum. FIG. 3 illustrates a pulse 10 which is used to fire a transducer. The pulse 10 causes the transducer to produce an ultrasonic signal 17. As shown, the ultrasonic signal 17 has a rise period 11 before reaching its peak 12, and then a fall period 13 to a static level. This phenomenon is due largely to the inertia of the diaphragms in both the transmitting and receiving transducers. They cannot be brought to their maximum displacement immediately upon application of the firing pulse. In the same manner, the toneburst does not automatically cease upon the termination of electrical impulses. The amplitude generally decays in an exponential manner.
The known ultrasonic ranging methods are inadequate for mining industry requirements. The most common ranging method in pulse-echo ranging illustrated in FIG. 3. In this method a transmitting transducer is fired and the resulting ultrasonic signal 17 propagates away from the transducer, is reflected from an object, and is then received by the receiving transducer in an attenuated form 14. The time difference between the start of the signal at the transmitting transducer and the reception of the reflected signal is used to give an estimate of the distance between the transducer and the object. The receiving transducer is triggered when a pulse is received which is greater than a predetermined threshold 15. This is usually achieved within the first few wavefronts of the ultrasonic signal. As described above, the longer the distance travelled, the larger the attenuation of the received signal. Since the amplitude of the received signal is compared to a set threshold, and the received signal takes a number of cycles to build up to its maximum amplitude, the receiver will be triggered on different wavefronts, depending on the distance and reflection properties of the reflecting object. This results in a significant error in the measurement of the distance between the transducers and the object.
The variable gain method is a form of pulse echo ranging which partially reduces this error by varying the threshold or the gain of the receiving amplifier over time. By increasing the gain of the received signal over time, the attenuation is counteracted. This method alleviates the above error, but is not able to account for variations in intensity from other sources of amplitude variation, such as angle deviation errors and changes in reflection properties.
Another known method is the modulated carrier method. In this method an ultrasonic wave is continuously transmitted and is modulated by a lower frequency wave. The phase differences of the low frequency components at the transmitter and receiver are examined and is directly proportional to the straight line distance plus a constant. However, the method only has a usable distance measurement range of one wavelength of the low frequency component. This is always less than one meter in magnitude.
The final known ranging method uses linear frequency modulation (LFM). In this method a chirp is transmitted. The frequency is varied in a linear sweep from low frequency to high frequency. This chirp is received and mixed with the transmitting waveform to produce a relatively constant, difference frequency. The difference frequency is proportional to the distance measured. The main problems involved with this method is the limited bandwidth and non-linearity of the transducers resulting in reduced accuracy.
Further errors in measurement can occur due to false triggering from ultrasonic noise, constructive and destructive interference between the source and reflected ultrasonic waves, transducer bandwidth limitations and changes in velocity of the ultrasonic waves due to air temperature changes. In many applications, such as mining environments, further errors occur due to temperature changes in the air over the distance being measured and over time. Any reliable method for compensating a measurement system for changes in the speed of sound due to temperature variations may be used. A temperature transducer such as a thermistor or thermo-couple can be used. The correct speed can then be calculated for the temperature of the environment when the measurement is made. Such a configuration was discussed ablove and shown in FIG. 1. Whilst this method is desirable in its simplicity, a significant problem arises if the air temperature is not homogeneous. That is, if the temperature varies significantly over the distance being measured, then significant errors can result in the measured distance.
The present invention attempts to overcome one or more of the above problems.